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For Iran, Enriching Uranium Only Gets Easier

President Mahmoud Ahmadinejad of Iran touring the Natanz nuclear site.

In the Iranian desert, at a sprawling industrial site ringed by barbed wire and antiaircraft guns, a shift in the enrichment of uranium is producing global jitters because it could shorten Iran’s path to the acquisition of nuclear weapons.

It is also illustrating one of the peculiarities of uranium enrichment, a version of the rich getting richer, really fast. The tricky process accelerates as it moves ahead. “The higher the concentration, the easier it gets,” said Houston G. Wood III, a professor of mechanical and aerospace engineering at the University of Virginia who specializes in nuclear enrichment. The process is, as scientists like to say, nonlinear.

Four years ago, Iran began enriching uranium on an industrial scale with centrifuges, machines that spin extraordinarily fast to separate uranium 235 from the more common form of the element, uranium 238. Uranium 235 is a natural rarity that splits easily in two, or fissions, in bursts of atomic energy, either in a reactor or a bomb. Reactor-grade fuel is usually defined as uranium 235 of about 4 or 5 percent, and bomb-grade as 90 percent or higher.

The desert complex, the Natanz nuclear facility, raised the level of uranium 235 to roughly 4 percent from its natural concentration of 0.7 percent. Over time, the facility produced two tons of concentrated material, enough, if further enriched, to make about two atom bombs.

Then, on Sunday, Feb. 7, Iran announced it would begin enriching its stockpiled uranium to 20 percent — ostensibly to make fuel for a research reactor in Tehran. Nuclear experts said that although this might sound like a leap, moving to 20 percent from 4 percent was actually a fairly easy step — not at all as demanding and time consuming as raising the level to 4 percent from 0.7 percent. And the ease of further enriching uranium once it is already enriched made the world take notice.

The new enrichment effort began Tuesday, Feb. 9. Three days later, France, Russia and the United States criticized Iran’s move as an “escalation.” In a letter to the International Atomic Energy Agency, an arm of the United Nations in Vienna, the three United Nations ambassadors from those countries said the heightened effort raised “new concerns about Iran’s nuclear intentions.”

A senior Obama administration official, speaking on the condition of anonymity, told reporters on Feb. 18 that the enrichment step and related Iranian moves were clear provocations. “This program,” the official said, “is heading more and more in the direction of seeking a weapons capability.”

In scientific terms, rather than political ones, the issue is “nonlinearity,” a word almost as scary to nonscientists as highly enriched uranium is to the International Atomic Energy Agency. The definition is fairly simple, however. In a nonlinear process, a particular input produces an output that is disproportional in size, either big or small. The weather is conspicuously nonlinear, with small changes in one area producing deadly storms elsewhere. So are earthquakes and explosions. The word describes inequality in cause and effect.

A practical illustration of nonlinearity is that Iran — or any other nuclear hopeful — needs increasingly few centrifuges to make uranium 235 increasingly potent. For instance, one industry blueprint features 3,936 centrifuges for enriching up to 4 percent, 1,312 centrifuges to 20 percent, 546 centrifuges to 60 percent and just 128 centrifuges to 90 percent — the level needed for a bomb.

The reason is that “you’re moving a lot more material at lower levels of enrichment,” said David Albright, president of the Institute for Science and International Security, a private group in Washington that tracks nuclear proliferation and disclosed the blueprint. “It’s the reduction of the material” that makes the process gradually easier.

Centrifuges that whirl small amounts of gaseous uranium at high speeds are used to separate the element’s different forms, or isotopes, starting with material that consists of 99.3 percent of the heavier uranium 238. At each step, more of the heavy uranium is removed and the remaining material, now with a higher concentration of the lighter isotope, goes through the centrifuge process again.

Uranium ore has about 140 atoms of the heavy isotope for every light one, and separating the two takes a lot of spinning. By the time the enrichment process has reached 4 percent, it has successfully removed some 115 of the heavy atoms.

To get from there to 20 percent — the enrichment goal of the Iranians — the spinning centrifuges need remove only 20 more of the heavy atoms. And from there it is even easier to jump to 90 percent, bomb grade, by removing four or so additional heavy atoms. That is what worries many countries.

In the desert, at the Natanz complex, Iran presented atomic inspectors with evidence that it had succeeded in enriching some of its 4 percent uranium to 20 percent, the United Nations agency said in a Feb. 18 report. But American and European officials said the amounts were small so far.

Originally, Iran enriched its uranium to 4 percent with thousands of centrifuges in two cavernous underground halls roughly half the size of the Pentagon. The center of its new effort, according to the atomic agency, is a facility at Natanz known as the pilot plant, where Iran currently has 164 centrifuges spinning. Even with the aid of nonlinearity, that number is insufficient to enrich much uranium quickly.

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In interviews and briefings, officials in Washington and diplomats in Europe said the pilot plant could make perhaps three kilograms, or about seven pounds, of 20 percent fuel per month. At that rate, they added, making enough to power the research reactor in Tehran would take five to seven years. But the reactor has only months to go before it could run out of fuel, they estimated.

The experts said the leisurely enrichment pace suggested that Iran’s declared goal was disingenuous and that its real motive was simply to escalate its defiant brinkmanship and up the ante in global negotiations over its nuclear program. Moreover, the enriched material must be turned into reactor fuel rods — a process that many experts doubted Tehran could master.

Iranian officials have all but admitted that the aim of the enrichment move — at least in part — is to strengthen its negotiating stance. Last week, when reporters asked Ali Asghar Soltanieh, Iran’s ambassador to the International Atomic Energy Agency, when the country expected to produce the fuel assemblies, he professed to have no idea.

He added, “We have opened a window of opportunity for the others to prove their political will to come and to have a deal on the nuclear fuel.” For months, Iran and the West have negotiated unsuccessfully to have Iran’s 4 percent uranium sent abroad for processing into reactor rods. The West wants the material out of Iran to preclude its transformation into bomb fuel.

Since 2006, the United Nations Security Council has repeatedly called on Iran to halt its uranium enrichment. Washington and its allies say that Tehran is seeking the ability to make nuclear weapons, a charge it strongly denies.

The Institute for Science and International Security, in a March 3 report, called Iran’s enrichment to 20 percent “particularly troubling.” As early as next year, it said, Iran could accumulate enough material to take the next step on the path of centrifuge concentration and make bomb-grade fuel. If Iran fed the 20 percent material back into thousands of spinning centrifuges at its main plant, the institute said, it could make sufficient fuel for a weapon “within a month.”

Iran’s sprawling enrichment plant at Natanz currently houses more than 8,000 centrifuges and is designed to hold 54,000 — the number is that large because of Iran’s ostensible goal of supplying reactors with tons of fuel.

Another worry for the West is whether Iran is developing a new clandestine site to further enrich its uranium. Last September, President Obama disclosed the existence of a half-built plant buried inside a mountain near the holy city of Qum. The plant is designed to hold 3,000 centrifuges, but experts said its unveiling had ruined its original purpose as a secret site.

Iran is now disconnecting hundreds of centrifuges at the main plant at Natanz, according to the atomic inspectors. Experts were unsure why. The move could signal technical troubles or new plans.

The institute, in its report, suggested that “Iran could be in the process of moving at least some of these centrifuges elsewhere,” perhaps to new clandestine sites. Iran last year announced plans to build 10 centrifuge plants underground to protect them from aerial attack. Experts agreed that it stood little chance of making 10 plants, but might succeed in building 1 or 2.

All told, there are 20 operating enrichment plants in the world, according to a recent study by Oak Ridge National Laboratory in Oak Ridge, Tenn.

Nuclear specialists said that the vast majority of those plants — unlike Iran’s — enrich their uranium to no more than 4 or 5 percent. Despite the technical ease of further enriching uranium into fuel for a bomb, said Mr. Albright, the Institute for Science and International Security’s president, “there are very few that go to weapon grade.”

A version of this article appears in print on March 9, 2010, on Page D1 of the New York edition with the headline: Enriching Uranium Only Gets Easier. Order Reprints|Today's Paper|Subscribe